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This invention relates to optical channel monitors, and more particularly to such devices utilizing a linearly variable filter (LVF).
The evolution of optical telecommunication systems to dynamically controlled wave-division multiplexing (WDM) networks has created a strong demand for optical channel monitoring devices. An optical channel monitoring device typically measures wavelength, power, and optical signal-to-noise ratio of each wavelength channel. It thus enables sophisticated and accurate control of the network. Some of the typical performance requirements for the state-of-the-art optical channel monitoring devices are wavelength channel resolution of 0.2 nm to 0.8 nm, wavelength accuracy of 50 to 100 pm, power accuracy of 0.5 dB, and optical signal-to-noise ratio measurement of up to 30 dB.
An optical channel-monitoring device typically consists of an optical system, electronics, and software. The function of the optical system resembles that of a spectrometer. That is, the optical system decomposes the incoming signal into wavelength or spectrum components using a dispersive element. Two types of dispersive elements that have been widely used for this purpose are gratings and Fabry-Perot etalons. In either case, the measurement is rather sensitive to a change in mechanical alignment. It is therefore a challenge to maintain required performance over long term through severe environmental changes that are assumed in most of the telecommunication applications.
Linear variable filter (LVF) is yet another type of dispersive element that has been used in the field of spectroscopy. LVF is made by depositing optical thin-film layers on a substrate in such a way that the thickness of the films varies linearly with position. The thickness variability is very small, of the order of a few microns over a few inches, or even less. The filter can be designed either as bandpass filter or high/low-cut filter. More details about LVFs can be found e.g. in U.S. Pat. No. 6,057,925 to Anthon, incorporated herein by reference. Spectral information of incoming optical signal can be obtained by placing a detector array behind the LVF (U.S. Pat. No. 5,166,755 issued to Gat, incorporated herein by reference). This approach enables rather compact and rugged mechanical design.
The use of the linear variable filter in optical channel monitoring devices, however, has been hindered by packaging problems. In particular, it has been a challenge to design a device that has sufficiently low cross talk between wavelength channels. The cross talk can be minimized by having a detector array that has much larger number of pixels than the number of channels and by making the width of the optical beam spot on the detector about as small as the pixel width. On the other hand, the width of the detector pixels in general decreases with the number of pixels in order to keep the practical size of the detector element. For example, the pixel width of the state-of-the-art 512-pixel detector array is about 25 xcexcm. This poses a challenge in packaging. Assuming Gaussian profile of the beam, the depth of focus that is defined by Rayleigh range is only 0.3 mm for the beam diameter of 25 xcexcm at the wavelength of 1.55 xcexcm (B.E.A. Saleh and M.C. Teich, xe2x80x9cFundamentals of Photonics,xe2x80x9d John Wiley and Sons, (New York 1991), pp.86-87).
On the other hand, it is not practical to place LVF at close proximity to the detector array for a number of reasons. Depositing LVF coating on the surface of the detector array is difficult because of the delicate surface and wiring of the detector array. Placing a separate LVF element inside the detector package is also problematic since it requires the removal of a window plate that is part of a hermetic package that protects the delicate detector surface. In addition, the need for minimizing the package size of the device sometimes requires freedom to place LVF more than several millimeters away from the detector array package.
In U.S. Pat. No. 6,057,925, supra, now assigned to the same corporate assignee as the present invention, Anthon discloses the use of micro lens array between the LVF and the detector array. The Anthon method enables focusing of the optical beam on the detector array while LVF is placed at an arbitrary position. However, the introduction of the micro lens array will cause a substantial increase in packaging cost. In addition, light scattering and/or aberration around the boundary of each lens are potential problems that may increase the cross talk between wavelength channels.
Accordingly, there is a need for an optical channel-monitoring device that overcomes the above problems.
In accordance with the invention, there is provided an optical channel monitoring device comprising: an input port for launching a beam of light, a linear variable filter disposed in the path of the beam of light for selectively transmitting light in a variable manner along a length of the filter, a detector means for measuring spectral characteristics of the light transmitted through the LVF, the detector means comprising a photodetector array disposed in the path of light transmitted through the LVF in a predetermined position relative to the LVF, and collimating means disposed between the input port and the LVF for collimating said beam of light. The collimated beam of light is incident on the LVF at a negative incidence angle selected to optimize focusing of the transmitted light on the photodetector array. In an embodiment of the invention, the LVF has a wedged layer and the collimated beam of light is incident on the LVF at an angle xcex80 determined according to the formula       Z    =                  -                  λ                      Sn            2                              ⁢              (                              θ            0                    +                      Δ            ⁢                          xe2x80x83                        ⁢            θ                          )              ,
where Z is focus position on the photodetector array, xcex is wavelength, S is wavelength slope of the LVF, n is the effective refractive index of the LVF, xcex80 is incidence angle of the collimated light beam on the LVF, and xcex94xcex8 is the half divergence angle of the output light beam from LVF.